January 1 is when people decide to get back into shape. It is wonderful for the income of gyms, because people pay up and last a few months.
All the same, my general advice to those over 35 or 40 is to start slowly. If overweight, tiny meals. If underweight, eat food of any sort. Begin with 3 hrs/wk at the gym for a month to get going.
Best way to do it is with a trainer.
I suggest two half-hours of light cardio, 2 half-hours of light weights/machines, and 2 half-hours of calisthenics. After that, a more demanding program.
Most people can use a guide through the journey of physical fitness. It is a lifetime journey to try to avoid eventual decrepitude.